Some elements of the fundamentals of chemical thermodynamics begin to be considered in high school. In chemistry lessons, for the first time, students come across such concepts as reversible and irreversible processes, chemical equilibrium, thermal effect, and many others. From the school physics course, they learn about internal energy, work, potentials, and even get acquainted with the first law of thermodynamics.
Definition of thermodynamics
Students of universities and colleges of chemical engineering speci alties study thermodynamics in detail within the framework of physical and/or colloidal chemistry. This is one of the fundamental subjects, the understanding of which allows you to perform the calculations necessary for the development of new technological production lines and equipment for them, solving problems in existing technological schemes.
Chemical thermodynamics is usually called one of the branches of physical chemistry that studies chemical macrosystems and related processes based on the general laws on the transformation of heat, work and energy into each other.
It is based on three postulates, which are often called the principles of thermodynamics. They do not havemathematical basis, but are based on the generalization of experimental data that have been accumulated by mankind. Numerous consequences are derived from these laws, which form the basis of the description of the surrounding world.
Tasks
The main tasks of chemical thermodynamics include:
- a thorough study, as well as an explanation of the most important patterns that determine the direction of chemical processes, their speed, the conditions that affect them (environment, impurities, radiation, etc.);
- calculation of the energy effect of any chemical or physico-chemical process;
- detection of conditions for the maximum yield of reaction products;
- determination of criteria for the equilibrium state of various thermodynamic systems;
- establishing the necessary criteria for the spontaneous flow of a particular physical and chemical process.
Object and object
This section of science does not aim to explain the nature or mechanism of any chemical phenomenon. She is only interested in the energy side of the ongoing processes. Therefore, the subject of chemical thermodynamics can be called energy and the laws of energy conversion in the course of chemical reactions, the dissolution of substances during evaporation and crystallization.
This science makes it possible to judge whether this or that reaction is capable of proceeding under certain conditions precisely from the energy side of the issue.
The objects of its study are called heat balances of physical and chemical processes, phasetransitions and chemical equilibria. And only in macroscopic systems, that is, those that consist of a huge number of particles.
Methods
Thermodynamic section of physical chemistry uses theoretical (calculation) and practical (experimental) methods to solve its main problems. The first group of methods allows you to quantitatively relate different properties, and calculate some of them based on the experimental values of others, using the principles of thermodynamics. The laws of quantum mechanics help to establish the ways of describing and features of the motion of particles, to connect the quantities that characterize them with the physical parameters determined in the course of experiments.
Research methods of chemical thermodynamics are divided into two groups:
- Thermodynamic. They do not take into account the nature of specific substances and are not based on any model ideas about the atomic and molecular structure of substances. Such methods are usually called phenomenological, that is, establishing relationships between observed quantities.
- Statistical. They are based on the structure of matter and quantum effects, allow describing the behavior of systems based on the analysis of processes occurring at the level of atoms and their constituent particles.
Both of these approaches have their advantages and disadvantages.
| Method | Dignity | Flaws |
| Thermodynamic |
Due to the biggenerality is quite simple and does not require additional information, while solving specific problems |
Does not reveal the process mechanism |
| Statistical | Helps to understand the essence and mechanism of the phenomenon, since it is based on ideas about atoms and molecules | Requires thorough preparation and a large amount of knowledge |
Basic concepts of chemical thermodynamics
A system is any material macroscopic object of study, isolated from the external environment, and the boundary can be both real and imaginary.
Types of systems:
- closed (closed) - characterized by the constancy of the total mass, there is no exchange of matter with the environment, however, energy exchange is possible;
- open - exchanges both energy and matter with the environment;
- isolated - does not exchange energy (heat, work) or matter with the external environment, while it has a constant volume;
- adiabatic-isolated - does not have only heat exchange with the environment, but can be associated with work.
The concepts of thermal, mechanical and diffusion contacts are used to indicate the method of energy and matter exchange.
System state parameters are any measurable macrocharacteristics of the system state. They can be:
- intense - independent of mass (temperature, pressure);
- extensive (capacitive) - proportional to the mass of the substance (volume,heat capacity, mass).
All these parameters are borrowed by chemical thermodynamics from physics and chemistry, but acquire a slightly different content, since they are considered depending on temperature. It is thanks to this value that the various properties are interconnected.
Equilibrium is a state of a system in which it comes under constant external conditions and is characterized by a temporary constancy of thermodynamic parameters, as well as the absence of material and heat flows in it. For this state, the constancy of pressure, temperature and chemical potential is observed in the entire volume of the system.
Equilibrium and non-equilibrium processes
The thermodynamic process occupies a special place in the system of basic concepts of chemical thermodynamics. It is defined as changes in the state of the system, which are characterized by changes in one or more thermodynamic parameters.
Changes in the state of the system are possible under different conditions. In this regard, a distinction is made between equilibrium and non-equilibrium processes. An equilibrium (or quasi-static) process is considered as a series of equilibrium states of a system. In this case, all its parameters change infinitely slowly. For such a process to take place, a number of conditions must be met:
- Infinitely small difference in the values of acting and opposing forces (internal and external pressure, etc.).
- Infinitely slow speed of the process.
- Maximum work.
- An infinitesimal change in external force changes the direction of the flowreverse process.
- The values of the work of direct and reverse processes are equal, and their paths are the same.
The process of changing the non-equilibrium state of the system to equilibrium is called relaxation, and its duration is called relaxation time. In chemical thermodynamics, the greatest value of the relaxation time for any process is often taken. This is due to the fact that real systems easily leave the state of equilibrium with the emerging flows of energy and/or matter in the system and are non-equilibrium.
Reversible and irreversible processes
Reversible thermodynamic process is the transition of a system from one of its states to another. It can flow not only in the forward direction, but also in the opposite direction, moreover, through the same intermediate states, while there will be no changes in the environment.
Irreversible is a process for which the transition of the system from one state to another is impossible, not accompanied by changes in the environment.
Irreversible processes are:
- heat transfer at finite temperature difference;
- expansion of a gas in a vacuum, since no work is done during it, and it is impossible to compress the gas without doing it;
- diffusion, since after removal the gases will easily diffuse mutually, and the reverse process is impossible without doing work.
Other types of thermodynamic processes
Circular process (cycle) is such a process, duringwhich the system was characterized by a change in its properties, and at the end of it returned to its original values.
Depending on the values of temperature, volume and pressure characterizing the process, the following types of process are distinguished in chemical thermodynamics:
- Isothermal (T=const).
- Isobaric (P=const).
- Isochoric (V=const).
- Adiabatic (Q=const).
The laws of chemical thermodynamics
Before considering the main postulates, it is necessary to remember the essence of the quantities characterizing the state of various systems.
The internal energy U of a system is understood as the stock of its energy, which consists of the energies of motion and interaction of particles, that is, all types of energy except for kinetic energy and its potential energy of position. Determine its change ∆U.
Enthalpy H is often called the energy of the expanded system, as well as its heat content. H=U+pV.
Heat Q is a disordered form of energy transfer. The internal heat of the system is considered positive (Q > 0) if heat is absorbed (endothermic process). It is negative (Q < 0) if heat is released (exothermic process).
Work A is an ordered form of energy transfer. It is considered positive (A>0) if it is performed by the system against external forces, and negative (A<0) if it is performed by external forces on the system.
The basic postulate is the first law of thermodynamics. There are manyhis formulations, among which the following can be distinguished: "The transition of energy from one type to another occurs in strictly equivalent quantities."
If the system makes a transition from state 1 to state 2, accompanied by the absorption of heat Q, which, in turn, is spent on changing the internal energy ∆U and doing work A, then mathematically this postulate is written by the equations: Q=∆U +A or δQ=dU + δA.
The second law of thermodynamics, like the first one, is not derived theoretically, but has the status of a postulate. However, its reliability is confirmed by the consequences of it corresponding to experimental observations. In physical chemistry, the following formulation is more common: "For any isolated system that is not in a state of equilibrium, the entropy increases with time, and its growth continues until the system enters a state of equilibrium."
Mathematically, this postulate of chemical thermodynamics has the form: dSisol≧0. The inequality sign in this case indicates the non-equilibrium state, and the "=" sign indicates equilibrium.
